1. Field of the Invention
The present invention relates to dielectric ceramic compositions for use as resonators employed in a microwave frequency band of several gigahertz.
2. Description of the Prior Art
The recent increase in information density results in a progressively increased signal frequency used. Particularly, a microwave having a frequency of several hundred megahertz to several gigahertz is used in various information transmission media such as satellite communication, microwave remote communication, broadcasting, and further a microwave remote recognition system.
A resonator or a filter for a transmitter-receiver is indispensable in such media, and is manufactured using a dielectric ceramic material adequately performing a function in its high frequency band.
Examples of this type of dielectric ceramic material conventionally used include dielectric ceramic materials of a BaO--TiO.sub.2 system, a Ba {Zn.sub.1/3 (Nb.Ta).sub.2/3 } O.sub.3 system, a (Zr.Sn) TiO.sub.4 system, and the like for the reason that the high frequency characteristics are relatively good.
In the resonator or the like made of the dielectric ceramic material, however, if the dielectric constant of its dielectric is taken as .epsilon., the wavelength of an electromagnetic wave propagating through the dielectric is small, i.e., 1/.sqroot..epsilon.. Consequently, the higher the dielectric constant .epsilon. of the material used is, the smaller the size of the resonator or the like can be.
However, the dielectric constant of the above described dielectric ceramic material is generally low, i.e., 20 to 40. Accordingly, the size of the resonator is increased in a microwave frequency band of 1 to 3 GHz.
On the other hand, examples of a material having a high dielectric constant include SrTiO.sub.3 (.epsilon.; about 300) and CaTiO.sub.3 (.epsilon.; about 180). However, the temperature coefficients of resonance frequency .epsilon.f of the materials are respectively very high, i.e., +1700 ppm/.degree.C. and +800 ppm/.degree.C., so that the stable use cannot be expected.
Examples of a method for bringing the temperature coefficient .tau.f of such a dielectric composition near zero include a method of mixing a dielectric ceramic material having a high dielectric constant and having a temperature coefficient .tau.f which is large on the positive side and a dielectric ceramic material having a high dielectric constant and having a temperature coefficient .tau.f which is large on the negative side.
However, a material having a high dielectric constant .epsilon. generally has a temperature coefficient .tau.f which is large on the plus side. Accordingly, it is impossible to find a suitable material having a high dielectric constant and having a temperature coefficient .tau.f which is minus, and it is difficult to make the Q value of the material larger as a microwave dielectric ceramic material.
In recent years, therefore, as a material solving the problems, a dielectric ceramic composition expressed by a composition formula of w.LiO.sub.2 -x.CaO-y.D.sub.2 O.sub.3 -z.TiO.sub.2 (where D is Sm or Nd) which is disclosed in U.S. Pat. No. 5,188,993 . It is known that with respect to the dielectric ceramic material, good dielectric properties are obtained when the ranges of w, x, y and z are respectively 0.0 mole % &lt;w.ltoreq.25.0 mole %, 0.0 mole % .ltoreq.x.ltoreq.50.0 mole %, 0.0 mole % &lt;y.ltoreq.20.0 mole % and 0.0 mole % &lt;z.ltoreq.80.0 mole %, where w+x+y+z=100 mole % in the above described composition formula.
In a dielectric ceramic composition in a case where D is Nd in the above described composition formula, that is, a dielectric ceramic composition expressed by a composition formula of w.LiO.sub.2 -x.CaO-y.Nd.sub.2 O.sub.3 -z.TiO.sub.2, it becomes clear that good dielectric properties are obtained even if a mixture ratio of Nd is more than 20 mole % and not more than 30 mole %, that is, 0.0 mole % &lt;y.ltoreq.30.0 mole %, and it can be expected that the same effect is obtained even if Nd is replaced with Sm.
Although the above described composition expressed by a composition formula of a w.LiO.sub.2 -x.CaO-y.D.sub.2 O.sub.3 -z.TiO.sub.2 (where D is Sm or Nd) exhibits superior properties to the conventional materials, a further improvement in the properties has been desired.